In the control of an AC rotating machine, a speed sensor or a position sensor is normally used in order to rotate the rotating machine at a desired output or rotational speed. This method involving attachment of these sensors, however, increases the cost and deteriorates performance due to wiring. Hence, there is a problem that this method is disadvantageous in fault tolerance and maintenance. To overcome this problem, there are proposed methods of detecting a magnetic-pole position and a rotational speed of the AC rotating machine without using a sensor.
Of these methods, there is a method using an inductive voltage and this method is chiefly advantageous in an operation in a high-speed region in which the inductive voltage is high. Meanwhile, for a speed region in which it is difficult to use an inductive voltage, such as a zero speed or low speed region, there is a technique of estimating a magnetic-pole position using saliency of an inductance by superimposing a voltage or a current at a frequency different from a fundamental frequency on the AC rotating machine.
For example, the invention described in PTL 1 discloses a method of estimating a magnetic-pole position by applying a high-frequency alternating voltage to the AC rotating machine so that an amplitude of a high-frequency current flowing in an orthogonal direction of the applied voltage becomes 0.
The invention described in PTL 2 discloses an estimation method as follows. That is, a high-frequency current value obtained by applying a high-frequency alternating voltage to the rotating machine is transformed to a d-q axis coordinate with a 45° phase shift from an estimated angle. A magnetic-pole position is then estimated so that the high-frequency impedances Zdm and Zqm obtained from the transformation result coincide with each other. Further, a correction under high load is made by subtracting a compensation angle θ^r computed by multiplying a torque component of a current instruction value by a proportional constant from the estimated magnetic-pole position. An estimated position θ^c is thus computed.